Cellular turnover of the hematopoietic system is supported by a small population of cells termed hematopoietic stem cells(HSCs). The mechanisms underlying the proliferation and differentiation of HSCs are incompletely understood. To study the cell kinetics of HSCs, the AC133+ cells, a subpopulation of hematopoietic stem and progenitor cells were first exposed to an expansion-promoting liquid medium, then grown in presence of either of the lineage specific cytokines EPO or G-CSF. The cells cultured for 14 days in liquid medium containing EPO were amplified 831-fold, and 98.2% of the resulting cells were erythroid. A similar culture exposed to G-CSF for 14 days grew 1350-fold, and 97.4% of progeny cells were myeloid. When the EPO-stimulated population was re-cultured with G-CSF, erythroid cells decreased gradually and myeloid cells developed, constituting 95.2% of the culture after 14 days; cell number grew 5075-fold over the primary inoculum. Conversely, reculture of the G-CSF-stimulated population with EPO resulted in 81.4% erythroid cells and 15.8% granulocytes after 14 days; cell number grew 4083-fold. When primary colonies were grown on semisolid medium containing EPO or G-CSF, then individually replated on medium containing the other cytokine, some secondary colonies showed self-renewal potential after 14 days and could be induced to develop into a different lineage. Analyses of cell surface antigens and receptors by single-cell RT?PCR and in situ hybridization showed that asymmetric cell divisions were occurring. CD34 and Notch1, EPO-R or CD13 were expressed within the same cells even after 28 days of culture. Our results indicate that progenitor cells co-express genes from different lineage pathways before commitment and that cytokines influence lineage commitment.